Re: [PATCH perf/core] perf intel-pt: Fix clang build failure in intel_pt_synth_pebs_sample

Ian Rogers
2020-05-14T19:07:06Z

On Thu, May 14, 2020 at 8:01 AM Gustavo A. R. Silva
<gustavoars@kernel.org> wrote:
>
> On Thu, May 14, 2020 at 10:10:30AM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Wed, May 13, 2020 at 06:47:38PM -0500, Gustavo A. R. Silva escreveu:
> > > Fix the following build failure generated with command
> > > $ make CC=clang HOSTCC=clang -C tools/ perf:
> > >
> > > util/intel-pt.c:1802:24: error: field 'br_stack' with variable sized type 'struct branch_stack' not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
> > > struct branch_stack br_stack;
> > > ^
> > > 1 error generated.
> > >
> > > Fix this by reordering the members of struct br.
> >
> > Yeah, I noticed that as far back as with ubuntu 16.04's clang:
> >
> > clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)
> >
> > util/intel-pt.c:1802:24: error: field 'br_stack' with variable sized type 'struct branch_stack' not at the end of a struct or class is a GNU
> > extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
> > struct branch_stack br_stack;
> > ^
> > 1 error generated.
> >
> >
> > Will fold this with the bug introducing the problem to avoid bisection
> > problems.
> >
>
> I agree. Also, the commit hash of the "Fixes" tag only applies to the
> perf/core branch and, I guess that might create confusion.

So while this fixes the warning I believe it breaks the intent of the code.

tools/perf/util/branch.h:
struct branch_stack {
u64 nr;
u64 hw_idx;
struct branch_entry entries[];
};

tools/perf/util/intel-pt.c:
struct {
struct branch_stack br_stack;
struct branch_entry entries[LBRS_MAX];
} br;

The array in br is trying to extend branch_stack's entries array. You
might have to do something like:

alignas(alignof(branch_stack)) char storage[sizeof(branch_stack) +
sizeof(branch_entry) * LBRS_MAX];
struct branch_stack *br = &storage;

malloc/free may be nicer on the eyeballs.

Thanks,
Ian

> Thanks
> --
> Gustavo